A high-voltage battery which is a driving source of a traveling motor is mounted in an electric car or a hybrid car, and a charging device for charging the battery is provided. The charging device generally includes a power factor correction circuit (hereinafter, referred to as a PFC circuit) which corrects a power factor of power supplied from an AC power source, a DC-DC converter which steps up or steps down an output voltage from the PFC circuit, and a control unit which controls the PFC circuit and the DC-DC converter.
When the charging device is connected to the AC power source, and charging is started under the control of the control unit, the PFC circuit and the DC-DC converter operate, and thus the battery is charged with a DC voltage which is output from the DC-DC converter. During the charging of the battery, the control unit monitors an output voltage, and performs feedback control on the PFC circuit and the DC-DC converter so that a value of the output voltage becomes a target value. In addition, there is a case where an output current is monitored, and feedback control is performed so that a value of the output current becomes a target value. When the charging of the battery is completed, the control unit stops the operations of the PFC circuit and the DC-DC converter, and the remaining electric charge of an output capacitor provided on an output side of the DC-DC converter is released. Due to discharge of the output capacitor, an electric shock caused by the remaining electric charge is prevented.
Generally, the DC-DC converter includes a single switching element or a plurality of switching elements. As a driving signal of the switching element, a pulse width modulation (PWM) signal is generally used. When the DC-DC converter starts an operation, there is a control method of gradually increasing an output of the DC-DC converter by gradually increasing a duty of the PWM signal applied to the switching element or gradually shifting a phase of the PWM signal. This control method is referred to as “soft start control” in the present specification. The soft start control is disclosed in, for example, International Publication No. WO 2011/129185.
In addition, when the operation of the DC-DC converter is stopped, there is a control method of gradually decreasing an output of the DC-DC converter by gradually decreasing a duty of the PWM signal applied to the switching element or gradually shifting a phase of the PWM signal. This control method is referred to as “soft stop control” in the present specification. The soft stop control is disclosed in, for example, Japanese Unexamined Patent Application Publication No. H04-069051.
Japanese Unexamined Patent Application Publication No. 2008-160967 and Japanese Unexamined Patent Application Publication No. 2002-262550 disclose a technique of a stop control of a DC-DC converter. In Japanese Unexamined Patent Application Publication No. 2008-160967, in a synchronous rectification type DC-DC converter, when power supply to a load is stopped, electric charge accumulated in an output capacitor is released to the ground via a switching element of a synchronous rectification circuit. In Japanese Unexamined Patent Application Publication No. 2002-262550, in a synchronous rectification type DC-DC converter, when power supply to a load is stopped, electric charge accumulated in an output capacitor is regenerated on an input power source side via an inductor and a field effect transistor (FET).
Japanese Unexamined Patent Application Publication No. 2011-182575, Japanese Unexamined Patent Application Publication No. 2012-090476, and Japanese Unexamined Patent Application Publication No. 2012-095511 disclose a power failure countermeasure in a power supply device. In Japanese Unexamined Patent Application Publication No. 2011-182575 and Japanese Unexamined Patent Application Publication No. 2012-090476, a preconverter is provided in a previous stage of a DC-DC converter, and thus a sudden drop of an input voltage does not occur during the occurrence of a power failure. In Japanese Unexamined Patent Application Publication No. 2012-095511, in a case where a bus voltage is equal to or lower than a reference voltage, an operation of a DC load is stopped, and thus drop of the bus voltage is minimized, and a rush current in power failure recovery is minimized by a current limiting resistor.